Ian O’Donnell, professor of criminology at University College Dublin, writes on the drop in Irish prisoner numbers.

Something strange is happening in Irish prisons. After years of extravagant growth, the numbers in custody have fallen. Statistics produced by the Irish Prison Service show a drop from a peak of 4,390 in 2011 to 3,660 this month.

The downturn places Ireland in a favourable position compared to other European countries. Only Germany, the Netherlands and Scandinavia have lower imprisonment rates.

Especially striking is the divergence from England and Wales, from which we inherited our legal system and where we have traditionally looked for much of our criminal law and policy. The imprisonment rate is almost twice as high on the other side of the Irish Sea.

The reasons for this welcome – and largely unnoticed – trend are not clear.

Despite the high level of anxiety regarding crime and punishment, we know surprisingly little about the justice system and how it operates. Where information does exist, its reliability is often questionable. The crisis of confidence in the Garda figures is the latest in a long list of examples.

Attention has repeatedly been drawn to the reactive and ill-informed nature of legislation and policymaking in the area. Research has been slow to emerge and the data deficit remains large.

What we do know is that committals to prison have increased, that fewer prisoners are being granted temporary release, and that there has been a steady growth in the number of long sentences imposed, including life terms.

These factors should inflate the prison population. But, paradoxically, the trend is in the opposite direction. What is going on?

Part of the explanation is that after much lobbying, children have at last been removed from the prison system. The continuation of the peace process in Northern Ireland has also led to a decline in the number of subversives in prison. There are fewer immigration detainees. But the numbers in these groups were always relatively modest and their further diminution is a small part of the explanation.

The picture is greatly distorted by the use of imprisonment for fine defaulters. This grew from (a little over) 1,000 committals in 2006 to (a little under) 10,000 in 2015, before falling back somewhat last year but significantly pushing up overall committal rates.

Fine defaulters do not spend long in prison but their incarceration is disruptive for them and their families, pointless from a rehabilitative point of view, and costly. It absorbs scarce criminal justice resources and generates no compensatory income; when the committal to prison takes place, the unpaid fine is wiped out.

The imprisonment of fine defaulters has long been a problem but it was exacerbated by the financial crisis that unfolded after 2008. Not only were people less able to pay fines due to their straitened circumstances but it was rational for them not to pay.

If someone has been in prison before and the prospect of a few hours in custody does not deter, it makes sense to allow financial penalties to accumulate and eventually to write them off in exchange for a day trip to prison.

Recent legislation should curtail the number of fine defaulters ending up in custody, hopefully to the point of irrelevance. When this group is removed from consideration, the pattern of committals under sentence looks very different. Rather than a substantial rise there has been a substantial fall.

Does this reflect a change in sentencing practice or a different mixture of cases coming before the courts? Has the judiciary become more lenient or is it dealing with fewer serious offenders? Frustratingly, the data does not exist to allow us attempt to answer these questions.

It is not just committals under sentence that are in decline. Remands in custody have fallen too. Is this because of changes in the crime rate or the likelihood of prosecution? Or because those who appear before the courts are less likely to interfere with evidence or witnesses, or to abscond, or, if charged with a serious offence, to commit another serious offence? The precise reason, or combination of reasons, is not known.

While recent years have seen encouraging developments that ran counter to expectations, careful empirical research is crucial to ensure the State can capitalise fully on these trends.

Prison is hugely expensive, often counterproductive, and necessary only for the most serious or persistent offenders. If we can learn to use it less and to focus our limited financial resources on victim support and crime prevention instead, it will be to everyone’s advantage.

If a fraction of the savings associated with falling prisoner numbers was directed towards a programme of research to deepen understanding and inform policy choices, we would be better able to make sense of what is going on and to plan for the future. In this way, a short-term gain might be converted into a sustainable strategy of penal reform.

It would be a matter of considerable regret if what might prove to be a once-in-a-generation opportunity were to be squandered.

This article first appeared in The Irish Times and is reproduced here with the kind permission of the author.